AnonAddy (free, open-source anonymous email forwarding service)
CounterMail
Ethermail
Fastmail
Firefox Relay (masks your true email address from public view)
Kolab Now
Mailbox.org
MailFence
Namecheap (They host business emails.)
Posteo
Proton (One Proton account gives access to ProtonMail, ProtonVPN, Proton Calendar, and ProtonDrive.)
ProtonMail Bridge (“An application available to all paid users that enables the integration of your ProtonMail account with popular email clients, such as Microsoft Outlook, Mozilla Thunderbird, or Apple Mail.”)
Reagan.com
Runbox
SimpleLogin (email aliases; powered by Proton)
Skiff (“private, end-to-end encrypted workspace”; now also offers end-to-end encryption Email)
Soverin
Startmail (same company that built the Startpage search engine)
Thexyz
Thunderbird
Tutanota
Vivaldi (web browser that includes email)
Zoho (They also have a Notepad alternative.)